Many substances absorb ultra violet or visible light due to their chemical composition. The absorption of light by substances has been used as the basis for detecting the presence of, and measuring the concentration of, such substances for many years. The concentration of the substance can be determined by use of the Beer Lambert Law:
A=Ebc
Where:
A is light absorbance;
E is the molar light absorbtivity with units of L mol−1 cm−1;
b is the light path length of the sample defined in cm; and
c is the concentration of the compound in solution, expressed in mol−1.
The Emax represents the maximum absorption of a substance at a given wavelength.
The UV region can be considered to consist of light of wavelength in the region of 1 nm to 400 nm, light of wavelength of 180 nm to 300 nm being known as ‘deep UV’.
Most analytical instruments for detecting substances which absorb in the deep ultra violet (UV) region use a mercury-lamp, deuterium lamp or xenon flash lamp as a light source. One example of such an instrument is a flow cell in which a solution containing one or more UV absorbing substances is passed between a UV light source (e.g. a mercury-lamp) and a UV detector (e.g. a photomultiplier or a photodiode) and changes in the intensity of UV light reaching the detector are related to the concentration of UV absorbing substances in the solution.
The detection of proteins, nucleic acids and peptides are of great importance in many sectors, including the environmental, biological and chemical sciences. Proteins have mainly two absorption peaks in the deep UV region, one very strong absorption band with a maximum at about 190 nm, where peptide bonds absorb, and another less intense peak at about 280 nm due to light absorption by aromatic amino acids (e.g. tyrosine, tryptophan and phenylalanine)
Nucleic acids absorb UV light at around 260 nm, some of the subunits of nucleic acids (purines) having an absorbance maximum slightly below 260 nm while others (pyrimidines) have a maximum slightly above 260 nm.
Almost all proteins have a maximum absorbance at about 280 nm due to the content of the light absorbing aromatic amino acids. The light source in the detectors of analytical systems used to detect and measure protein concentrations has historically been the mercury-line lamp. Mercury produces light with a wavelength of 254 nm but not at 280 nm, so a fluorescence converter is needed to transform the 254 nm light produced by the mercury lamp to longer wavelengths and a band pass filter is used to cut out a region around 280 nm. Mercury lamps have relatively short lifetimes and can prove unstable with time; furthermore, the disposal of these lamps can lead to environmental problems. The other lamps used to generate ultra violet light, such as the deuterium and the xenon flash lamps, disadvantageously require high voltages, need complicated electronics and often prove unstable with time. All of the currently used ultra violet light sources are relatively large and are consequently unsuitable for miniaturisation of analytical instruments. Moreover, all of the lamps generate significant amounts of heat due to the high voltages required for their operation.
Recently light emitting diodes (LED) of type AlGaN/GaN with emissions in the 250 nm to 365 nm range have been developed. Sensor Electronic Technology, Inc. (Columbia, S.C., USA) have pioneered the development and use of these UV light emitting diodes, particularly for irradiating and sterilising fluids such as biologically contaminated water (e.g. US 2005/0093485). Other groups have also employed UV light emitting diodes for water purification systems (e.g. Phillips Electronics, WO2005/031881).
Light emitting diodes (LEDs), which emit in the visible region of the spectrum, have been used for indirect photometric detection (Johns C., et al. (2004) Electrophoresis, 25, 3145-3152) and fluorescence detection of substances in capilliary electrophoresis (Tsai C., et al. (2003) Electrophoresis, 24, 3083-3088). King et al. (Analyst (2002) 127, 1564-1567) have also reported the use of UV light-emitting diodes which emit at 379.5 nm for indirect photometric detection of inorganic anions.
The use of deep UV light emitting diodes as light sources in detection systems for nucleic acids is disclosed in US2005/0133724. However, although detection systems employing LEDs are disclosed, there are no experimental data to indicate that the proposed systems were indeed successfully employed to measure nucleic acid levels in polymerase chain reaction assay. The system described would lack sensitivity, linearity, and dynamic range because there is no use of a band pass filter or a beam splitter and reference detector; LEDs are very sensitive to minute changes in temperature, changes of the order of one hundredth of a degree Centrigrade causing a drift in the baseline. Furthermore, the system lacks a band pass filter which acts to both narrow the bandwidth and block light in the visible region of the spectrum. A narrow bandwidth compared to the natural bandwidth of the sample, preferable a ratio of 1 to 10, provides a good linearity of the response and a broad dynamic range. (Practical Absorbance Spectrometry. Ed. A Knowles and C. Burgess, Chapman and Hall, New York).
JP2002005826 discloses a system for measuring ozone concentration. However, no experimental data that show the linearity and dynamic range are provided. The system uses a solid state emitter, which is composed of a diamond semiconductor thin film, to emit ultraviolet light with an emission peak of wavelength 240 to 270 nm. The emission spectrum at half value width of the UV peak is somewhat narrower than the half value width of the peak of the absorption spectrum of ozone (emission maximum approximately 254 nm). However, while this may be sufficient to measure ozone concentrations, the lack of a band pass filter which can reduce the band width to, for example, one tenth of the half value width of the ozone absorption peak will significantly reduce the linearity and dynamic range of the detector (Practical Absorbance Spectrometry. Ed. A Knowles and C. Burgess, Chapman and Hall, New York). This system also lacks a reference photo detector, so no measurement of the intensity of the emitted light is made. This means that compensation of variations of the emitted intensity due to changes in temperature is not possible.
WO2007/062800 (incorporated herein by reference), describes the use of a UV LED as a source of light for analysis of the concentration of a substance in a liquid sample, but it has been found that a broader spectrum of light is desirable in order to subject the sample to different wavelengths and thereby define a substance more accurately or more quickly, by its absorption characteristics at different wavelengths. However, known LEDs have only a limited light wavelength output range.
The present invention addresses the aforementioned problems with the currently available light sources used in analytical systems for detecting and/or for measuring the concentration of a substance in a solution.